Fabrication of conductor-clad composites using molding compounds and techniques

ABSTRACT

Conductor-clad composites are advantageously fabricated using molding compounds and associated processing techniques. The conductive cladding is applied and bonded to the composite during the molding process. The conductor-clad composite may be used as printed wiring board, and in this embodiment results in improved physical, chemical, mechanical, and electrical properties.

This application is a continuation of application Ser. No. 900,937 filedApr. 28, 1978, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention involves the fabrication of conductor-clad compositesusing molding compounds and associated molding techniques.

2. Description of the Prior Art

Despite the advent of integrated circuitry, the much larger scaleprinted wiring board technology is still in very substantial use inthose areas where miniaturization is not critical. A prevalent techniquefor fabricating copper-clad composites, especially those compositeswhich are used as printed wiring board, involves the manufacture ofresin-impregnated woven reinforcement. Subsequent to impregnation of thewoven reinforcement with an appropriate resinous compound, the materialis passed through a heated oven where the polymer in the resinouscompound is caused to partially crosslink thereby yielding a somewhattacky structure commonly referred to as a "prepreg". The prepreg is cutto appropriate size and a number of such sheets are pressed in asuitable open platen laminating press at pressures of from 15 to 300pounds per square inch (psi) and at temperatures of from 250-550 degreesFahrenheit for approximately an hour. The laminating process may involvesimultaneously the application of an appropriate conductive foil sheet(e.g., copper) to one or both sides of the substrate, upon which aprinted circuit may be subsequently defined. Any vapors that form duringthe laminating process may escape through the side openings in the openplaten press, and consequently, have little deleterious effect upon thebonding of the copper and its resultant smoothness. This laminatingtechnique requires a significant amount of time (approximately one hour)due to the lengthy layup and curing steps. Material costs, primarilyassociated with the woven glass reinforcement, and the lengthyprocessing time make the cost of the product prohibitive for manyapplications.

Suggestions for lowering the cost of copper-clad composites include theuse of bulk molding compound to form an appropriate substrate to which aconductive cladding is subsequently bonded (see, for example, InsulationCircuits, November 1976, page P-41). Such a process, however, retainsthe costly dual-step nature of the prior process (i.e., substratefabrication and subsequent bonding). In addition, use of the resultantconductor-clad composite as printed wiring board has been largelyunsuccessful, in part, for lack of proper physical, mechanical,electrical, and chemical properties.

The pultrusion process has recently been applied to the single-stepfabrication of copper-clad composites (see U.S. Pat. No. 4,012,267). Thegood quality of the copper bond in the pultrusion process is due partlyto the "open" nature of the pultrusion press which allows for venting ofvapors produced during curing. While the single-step pultrusion processis highly effective, cost reduction is still an object of significantpursuit.

SUMMARY OF THE INVENTION

This invention is a method of fabricating conductor-clad compositesusing polymeric molding compounds and associated molding techniques. Theconductive cladding is bonded to the composite in a singlemolding-bonding step. The single-step nature of this process, whencombined with the inherent cost reduction obtained through the use of"filled" molding compounds, results in a potential cost reduction ofapproximately 50 percent when compared to prior art copper-cladcomposites. Specific formulations may be advantageously used when theclad composite is to be applied to the fabrication of printed wiringboard. The use of this process in conjunction with such specificformulations results in a printed wiring board with physical,mechanical, electrical, and chemical properties equal to, or superiorto, that obtained in the prior art, while providing significant costreduction.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a schematic representation of a molding apparatus whichmay be utilized to fabricate the conductor-clad composite.

DETAILED DESCRIPTION

This invention is a process for fabricating clad composites usingpolymeric molding compounds and associated molding techniques.Conductor-clad composites fabricated using this process may beadvantageously utilized in the fabrication of printed wiring board.

A. The Molding Process

The inventive process involves molding an appropriate polymeric moldingcompound in a heated molding apparatus while simultaneously applying atleast one conductive foil to the compound. While the formulation of themolding compound may proceed by any one of a number of differentprocesses, the molding step is generally the same for all moldingcompounds. This molding step utilizes a standard press fitted with aheated compression mold familiar to those skilled in the art and shownschematically in the FIGURE. The compression mold comprises a heatedfemale member, or cavity, 10 which may be fitted with an appropriateheated male member, or core, 11. The heated cavity 10 is filled with acharge of the molding compound 12, and at least one conductive foil 13,subsequent to which the mold is closed. The closure of the pressproceeds at a controlled rate depending on the reactivity of thecompounds.

B. The Resin Formulation

While the inventive molding process is broad enough to encompass the useof a number of different molding compounds (e.g., sheet moldingcompound, bulk molding compound, thick molding compound), the followingdiscussion is presented in the context of a specific sheet moldingcompound formulation in order to more easily detail the various facetsof this invention.

Table I is an example of a typical sheet molding compound formulationand includes ranges for the various constituents of the formulationwhich are meant to be broad enough to encompass all molding compounds.Table II reviews the properties of a conductor-clad composite fabricatedaccording to the teachings of this invention. Following the tables is adiscussion of the various elements of the formulation.

                  TABLE I                                                         ______________________________________                                        FORMULATION OF MOLDING COMPOUND                                                             Formulation Range                                                             (pts by wt ±5%)                                                                        (pts by wt)                                         ______________________________________                                        Resin (polymeric portion)                                                                     65.0          60-80                                           Monomer         43.0          0.5-60                                          Low Profile Additive                                                                          35.0          15-40                                           Catalyst        1.5           0.5-3                                           Filler          150.0         100-200                                         Viscosity Control Agent                                                                       2.5           0-5                                             Reinforcement   85.0           20-250                                         ______________________________________                                    

                                      TABLE II                                    __________________________________________________________________________    PROPERTIES OF THE CLAD COMPOSITE                                              PROPERTY   TEST & CONDITION                                                                              RESULT                                             __________________________________________________________________________    Solvent Sensitivity                                                           Trichloroethylene                                                                        5 minutes in boiling vapor                                                                    No visible signs of                                Methylene Chloride                                                                       15 miuntes soak at room temper-                                                               solvent attack                                                ature                                                              Chemical Degradation                                                                     Exposed to chemicals used in                                                                  No visible signs of                                           printed wiring board                                                                          degradation                                                   processing                                                         Solder Dip Resistance                                                                    Immerse in 500 degrees F.                                                                     No blisters or                                                molten solder for 30 seconds                                                                  conductor lift                                     Water Absorption                                                                         24-hour immersion in tap water                                                                0.35 percent typical                                          at room temperature                                                                           0.70 percent maximum                               Combustibility                                                                           Oxygen index (ASTM 2863)                                                                      32 percent typical                                                            28 percent minimum                                            Underwriters Labs (#UL-94)                                                                    94V-0                                              Surface Roughness                                                                        Profilimeter    20μ inches maximum                              Peel Strength                                                                            90 percent peel at room                                                                       3 lbs/inch conductor                                          temperature     width typical                                                                 2 lbs/inch conductor                                                          width minimum                                      Barcol Hardness                                                                          Barcol Impressor                                                                              55 Barcol Hardness typical                                    Model GYZJ 934-1                                                                              40 Barcol Hardness minimum                         Flexural Strength                                                                        ASTM D790       23 × 10.sup.3 psi typical                                               17 × 10.sup.3 psi minimum                    Modulus of Elasticity                                                                    ASTM D790       1.7 × 10.sup.6 typical                                                  1.0 × 10.sup.6 minimum                       Impact Strength            5 foot pounds                                      Insulation Resistance                                                                    ASTM D257 (interlocking                                                                       After 4 days =                                                comb pattern exposed to                                                                       6.0 × 10.sup.5 megohms                                  95 degrees F. and                                                                             After 11 days =                                               90 percent humidity)                                                                          3.0 × 10.sup.5 megohms                                                  After 28 days =                                                               2.5 × 10.sup.5 megohms                       __________________________________________________________________________

1. Resin

In this invention the resin is usually a thermosetting polymeric resinalthough high temperature thermoplastics may be used. In order toeliminate blistering of the cladding during the molding process, it isadvantageous that the resin cure with minimal occurrence of condensationreactions. By-products which form during such reactions could stimulateblistering and degrade the product to a point where it would beundesirable for the application envisioned. Specifically, such reactionsmay result in reduced peel strength and degraded electrical properties.

Exemplary resins used in this invention include isophthalic anhydridebased unsaturated polyester resins, such as HATCO LB555-44 or PPGSelectron 50239; vinyl ester based resins, such as Dow Chemical, DER786; orthophthalic anhydride based unsaturated polyester resins, such asHATCO GR 63003; and bisphenol based polyester resins, such as ATLAC 382.(The commercial resins are exemplary formulations familiar to thoseskilled in the art, however, the generalized resin description issufficient to allow one skilled in the art to practice this invention.It should be noted that some of the commercially available resins mayinclude some monomer.) At the current time phenolic resins used inlaminating processes are known to cure only by means of a condensationreaction and during the molding process give off water vapor which couldcause blistering of the copper cladding and voids in the composite.Phenolic resins may, however, be used where the vapors formed during thecure are adequately vented. Epoxy resins which are thermosetting mayalso be used in this invention. The chemical engineering involved infabricating the resin is a highly complicated problem well known tothose skilled in the art.

2. Monomer

The monomer is a crosslinking member which forms a three-dimensionalnetwork with the polymeric portion of the resin formulation. Differentmonomers may be advantageously utilized depending upon the specificresin formulation used. The monomer also has the effect of reducing theviscosity of the formulation during the mixing process so as to increasethe subsequent wetting of the reinforcement material by the resinformulation. Typical monomers are styrene and diallyl phthalate. In thework described in this specification, styrene was widely used as themonomer.

3. Low Profile Additive

The low profile additive results in increased surface smoothness andincreased dimensional stability of the resultant composite. Generally,as the temperature increases during the molding process and the resincures, the resin shrinks around the glass reinforcement at thefiber-resin interface resulting in fiber prominences at the compositesurface. The low profile additive is incompatible with the resin atthese molding temperatures. Consequently, a phase separation occursduring whch time the generally thermoplastic low profile additiveseparates from the resin phase, absorbs a portion of the monomer, andexpands. It is theorized that the expanding low profile additive acts asa restraint to the shrinking resin. The resin, consequently, does notshrink around the fiberglas reinforcement, and reinforcement prominencesare minimized yielding the improved surface smoothness observed in thecomposite. Both polystyrene and polyvinyl acetate low profile additiveswere used during various phases of this work. In Example 1 a specificformulation is noted in which polyvinyl acetate is found to giveimproved results.

The glass transition temperature and the coefficient of thermalexpansion of the low profile additive may be advantageously chosen so asto maximize the additive's effect during the molding process. In Example1 this effect is maximized so as to yield particularly beneficialresults. However, different low profile additives may be selecteddepending upon the specific resin formulation utilized during thepractice of this invention.

4. Catalyst

The catalyst initiates crosslinking between the polymer and the monomer.The specific catalyst or catalysts which are used in a particularformulation is determined in part by the temperature which is selectedfor the molding process. In the work described in Example 1 bothtertiary butylperbenzoate, as well as a peroxyketal (Noury ChemicalTirgonox 29, i.e., 1,1 dibutylperoxy 3,5,5 trimethyl cyclohexane) wereused as catalysts. However, other catalysts, or mixtures of catalysts,may be used depending upon the temperature selected for use during themolding process.

5. Filler

The filler material is generally a material which does not play areactive part during the curing of the molding compound. It is selectedfor its low cost and for other characteristics which may improve theproperties of the resultant clad composite. In most instances the fillerwill be a nonpolymeric material.

Alumina trihydrate is a particularly beneficial filler in that itretards flammability. The alumina trihydrate used in Example 1 wastreated with a silane prior to its inclusion in the formulation in orderto maintain the resultant viscosity of the compound, which normallyincreases as the amount of alumina trihydrate is increased. The silanetreatment, and the resultant viscosity properties, permit better wettingof the reinforcement by the resin portion of the formulation andconsequently allows for the inclusion of greater amounts of filler thanwould normally be possible. Exemplary fillers include clays, calciumcarbonate, and reground or reprocessed thermosetting materials which mayhave been fully cured.

6. Viscosity Control Agent

The viscosity control agent increases the viscosity of the compound fromapproximately 3000 cps to 50 million cps during the maturation whichoccurs prior to molding. This need not be a crosslinking process, andthe viscosity control agent is generally not a crosslinking agent. Theprimary use of the viscosity control agent is to increase the viscosityof the compound so that it may be handled more readily prior toplacement in the mold. It is particularly beneficial in the sheetmolding process where sheets of the molding compound are formed forplacement into the mold as integral units. However, when bulk moldingcompound or thick molding compound is used and such handling of thematerial is not necessary the viscosity control agent may be removedfrom the formulation. Typical viscosity control agents include magnesiumoxide and magnesium hydroxide.

7. Reinforcement

The preferred reinforcement material is usually glass fiber generally ofa diameter of from 10-1000 microns. While the fiber reinforcement may becontinuous, typically it is not. Its length is dependent both upon theproperties which are desired for the ultimate clad composite and alsothe particular type of molding compound which is utilized. So forexample, the longer the fiber reinforcement the greater the strength ofthe resultant composite. While the sheet molding and the thick moldingcompound may contain glass fibers of from 1-4 inches, the bulk moldingcompound, by the nature of the mixing process, will result in glassfibers generally no longer than a quarter of an inch. Consequently, theuse of bulk molding compound may yield a composite which has lowerstrength than that obtained with the sheet molding and thick moldingcompounds.

The fiber, preferably prior to its being chopped, may be treated with acoupling agent so as to increase the bond between the resin and theglass fiber. Such a resin compatible treatment may include coating witha silane, with VOLAN or GARAN. The fiber reinforcement is not limited toglass fibers but may also include graphite fibers, carbon fibers,polyaromatic amid fibers, polyester fibers, cellulose fibers, or anyother appropriate reinforcement. In some sheet molding embodiments somecontinuous fiber strands may be used. In some embodiments in whichstrength is not critical the reinforcement may be entirely omitted.

C. The Molding Parameters

The molding parameters may be dependent on the size of the mold and sizeof the charge. However, generally the molding process involves moldingthe compound in a mold which has been brought to temperature prior toinsertion of the molding compound. As discussed above, since theinventive process yields a conductor-clad composite it includes placingat least one conductive foil into the mold prior to insertion of themolding compound into the mold. Molding temperatures may vary from200-500 degrees F., with typical values for molding compounds in theneighborhood of 285±30 degrees F. Pressure in the range from 100-1500psi are possible but are usually in the range of 800-1100 psi.

Since this is a rapid process typically with a cycle time of 1-5minutes, the rate of closure must be relatively rapid in order to ensureproper flow of material prior to curing. Closure usually occurs within aperiod of 15 seconds; and if a movement mold is used, any gases that areformed must be allowed to escape during this time since gases cannotescape once such a mold is closed. During the molding process (a periodgenerally of from 1-5 minutes) the mold remains heated; and while it maybe cooled down prior to removal of the molding compound, the mosteconomical embodiments involve retention of the mold at approximatelyfull heat during insertion and during subsequent removal of the curedcomposite. The composite is removed from the mold by means of anappropriate jet of air, knockout pins, or, in certain embodiments,through the use of additional suction cups.

Care should be taken while removing the composite to ensure that it isnot distorted. It may be desirable in certain embodiments to restrainthe composite in an appropriate fixture during cooling in order to limitdistortion of the composite. However, with the low profile additivessuggested in the example restraint of the composite subsequent tomolding is usually not necessary.

The size of the molding charge is usually on the order of 8 ounces persquare foot (for a 0.060 inch thick composite) and usually covers from30-90 percent of the mold surface and inserted. In commercialembodiments this number will usually be between 50 and 80 percent. Ithas been observed in this work that if the charge fills the entire areaof the mold, when inserted, then the inability of the charge to moveduring the molding process results in trapping of volatile products andresultant blistering of the clad composite. When sheet molding compoundis used, an appropriate charge configuration may be established byplacing a number of sheets in the press one on top of the other. Incertain embodiments the use of a pyramidal charge pattern may bepreferable.

The clad composite may be made with either one or two surfaces clad withan appropriate conductor such as copper. When copper is used the surfacewhich contacts the molding compound is generally pretreated to increasethe bond strength between the copper and the molding surface. Suchtreatment may include a brass flash, a zinc flash, an oxide treatment,or an appropriate adhesive treatment. It is found that if the copperfoil is the size of the mold surface area then expansion during themolding process results in severe wrinkling of the copper (i.e., foldson the composite). Consequently, it is important that the copper beapproximately centered in the mold and have linear dimensions (i.e.,length, width) at least one-hundredth of a percent less than theinternal linear dimensions of the mold face. Generally, the copper willbe between one-tenth of a percent and five percent less than the lineardimensions of the mold face. In commercial embodiments the lineardimensions of the foil will be between one percent and three percent ofthe mold face.

The copper is usually in the range from one-quarter to five-ounce copper(0.35 to 7 mils thick) and is usually either electro-deposited or rolledcopper. Lower thicknesses of copper, such as eighth-ounce copper, may beadvantageously utilized when, subsequent to the composite formation, acircuit is defined on the thin copper and then additional copper isdeposited electrolessly on the defined circuit to build up a layer ofcopper thick enough for commercial embodiments. In this specification,this process will be referred to as the partially additive process.

The tendency of the molding compound to adhere to the face of thecompression mold influences the operating parameters used in the courseof this invention. Two illustrative points involve (1) the fabricationof a composite with only a single cladding, and (2) the requirement ofan undersized cladding.

If the composite is fabricated with only a single cladding considerationmust be given to the tendency of the unclad side to adhere to the moldface during molding. In prior art applications in which the peelstrength of the single cladding is not critical, internal mold releasemay be used in the formation of the molding compound. Such a moldrelease significantly reduces the tendency of the molding compound toadhere to the mold. However, in view of the fact that the internal moldrelease also restricts the adherence of the cladding to the substrate,and consequently reduces its peel strength, such internal mold releasewill not generally be used in the practice of this invention.Alternative techniques applicable to this invention involve spraying anexternal mold release on the mold surface, or using an appropriateplastic film, such as Mylar, to prevent the deleterious adherence.

In a similar vein, if the conductive cladding--which is required to beless than the size of the interior mold face according to the teachingsof this invention--is made too small then the large amounts of exposedmolding compound will adhere to the mold face presenting significantdifficulties. This invention is properly practiced with a conductivecladding which, though undersized, is still sufficiently large so as tolimit the amount of molding compound exposed to the molding face therebyavoiding adherence of the molding compound to the mold face.

Subsequent to formation of the composite an appropriate electricalcircuit may be defined using techniques well known in the art.

D. Composite Characteristics

1. Surface Characteristics

Using this process, conductor-clad composites with desirablecharacteristics may be formed in a single molding step. For example, itis found that the surface characteristics of the printed wiring boardare improved over those available in the prior art. Specifically, thefiber prominences in the composite, when measured using a tallyprofilimeter, are less than 20 microinches rms and in many embodimentsless than 17.5 microinches rms. For comparison, conductor-cladcomposites with woven fiberglass reinforcement usually have surfacevariations greater than 20 microinches rms. Part of this improvement maybe attributed to specific properties of the invention described here.For example, a woven reinforcement is generally not used. Wovenreinforcement usually involves a cloth which has associated with itfiber prominences which appear at the fiber crossover points. Thisinvention utilizes unwoven chopped glass fiber less than four inches inlength which is embedded in the resin compound thereby avoiding anydeleterious effect on the surface characteristic of the subsequentprinted wiring board. In addition, low profile additives are used whichsignificantly further improve surface characteristics in the composite.

2. Glass Transition Temperature

The cured composite as described here has a high glass transitiontemperature, generally higher than 125 degrees C. The coefficient ofthermal expansion above the glass transition temperature should match asclosely as possible to that of the conductive cladding. The necessityfor this combination is that during thermal wave soldering the circuitis floated on 500 degrees F. molten solder thereby subjecting thecircuit to extreme thermal shock. If the coefficient of thermalexpansion of the substrate is significantly different than that of theconductor cracking in plated holes will occur. This is particularlycritical where holes are punched or drilled into the substrate andplated so that circuits on the front and back of the substrate may beconnected. Such holes are generally referred to as plated-through holesand can experience barrel cracking during solder float. There is a closethermal expansion match between the copper and the composite describedin the example.

3. Chemical Resistance

The inherent properties of the formulation as well as the high fillercontent yield excellent chemical resistance properties in the resultantboard. During printed circuit board processing, the printed wiring boardis exposed to various chemicals and solvents, such as ammoniumpersulfate, hydrochloric acid, trichloroethane and methylene chloride.When the printed wiring board is immersed in such chemicals for one-halfhour at room temperature no surface damage may be seen by the unaidedeye.

4. Moisture Absorption

Measurements on the weight of the substrate before and after 24-hourimmersion in water maintained at room temperature indicate very littlemoisture absorption (<1 percent). Consequently, when high temperaturesolder is exposed to the substrate, little or no blistering or conductordelamination occurs.

5. Insulation Resistance

The surface characteristics of the printed wiring board and its lowwater absorption makes for very high insulation resistance. Theinsulation resistance is measured between two conducting elements 0.025inches wide formed in an interlocking comb pattern with a combseparation of approximately 0.050 inches (ASTM D257) and is found to be6×10⁵ megohms after four days of exposure to an environment of 95degrees F. and 90 percent relative humidity and measured under theseconditions. This value is as good as the more costly epoxy laminatewiring board and is better than other polyester laminate wiring boards.It should be noted, however, that at the present time epoxy printedwiring boards cannot use large amounts of filler materials to decreasetheir cost of fabrication without deterioration of electrical andmechanical properties. In addition to the effect the smooth surface hason improving the insulation resistance of the printed wiring board, asmooth surface also results in lower cost when gold plating is utilizedsince gold fill-in on the surface may not be required.

6. Peel Strength

The peel strength on the printed wiring board is found to be greaterthan 2 pounds per inch of conductor width and is adequate for manyapplications.

7. Mechanical Strength

The flexural strength of the printed wiring board using this techniqueis 23,000 psi as compared to 75,000 psi for an epoxy glass substrate.However, the 23,000 psi flexural strength is more than adequate for mostapplications. The flexural modulus is found to be 1.7 million psi andthe impact strength 5 foot-pounds; both are more than adequate forenvisioned applications.

EXAMPLE 1

In this example a copper-clad composite was fabricated using the moldingtechnique described in the specification. The molding compound wasformulated using a sheet molding process. The components of theformulation are listed in Table III.

                  TABLE III                                                       ______________________________________                                                          pts by weight (±1%)                                      ______________________________________                                        Resin               65                                                        isophthalic anhydride based                                                   unsaturated polyester resin                                                   Low Profile Additive                                                                              35                                                        polyvinyl acetate                                                             Monomer             3                                                         styrene-viscosity reducing,                                                   crosslinking diluent                                                          Catalyst            1                                                         1,1 ditertiary butylperox                                                     3,3,5 dimethyl cyclohexane                                                    Filler              150                                                       alumina trihydrate                                                            treated with silane,                                                          acts as flame retardant                                                       Viscosity Control Agent                                                                           0.8                                                       magnesium oxide                                                               Chopped Glass Fiber Reinforcement                                                                 30                                                        approximately 12 microns                                                      diameter                                                                      ______________________________________                                    

The ingredients in Table III, with exception of the glass, were mixedinto a resin formulation using a Cowles-type mixer. The mixedformulation was then put into a holding tank. At this point theviscosity was approximately 3000 cps. From the holding tank, theformulation was metered onto a moving polyethylene film forming a layerapproximately one-eighth inch thick. One-inch glass rovings were thendeposited uniformly along the resin surface. Simultaneous to thisanother layer of resin formulation was metered onto a secondpolyethylene layer film. The polyethylene film with the chopped glassrovings then joined the polyethylene film without the chopped glassrovings to form a sheet molding compound layer comprising the choppedglass rovings sandwiched between the two films of resin formulation.This compound was then passed through kneading rolls where it was mixedwhile maintaining its sheetlike shape and then wound into a roll andsealed with polyethylene and aluminum foil. The roll was then placed ina maturation room.

The molding compound was kept in the maturation room at a temperature of95 degrees F. for approximately three days during which time it reacheda molding viscosity of approximately 60 million centipoise. Subsequentto reaching the maturation viscosity, the molding compound was cut intofive 10"×10" sheets with a total weight of 1000-1250 grams and thepolyethylene was removed. A layer of one-ounce copper (11/2 mils thick),whose linear dimension was approximately one-fourth inch less than thatof the mold, was placed in the bottom of the female member of a closedcompression molding apparatus with the treated side up. The mold was a24-square-inch flat compression mold. The five sheets of sheet moldingcompound were then placed upon the one-ounce copper inside the femalemember of the mold. An identical sheet of copper was then placed abovethe sheet molding compound with the treated side facing the sheetmolding compound. The sheet molding compound had a weight of 380 gramsper square foot. Prior to insertion of the sheet molding compound themold was heated to 280 degrees F. The mold was closed in a maximum of 15seconds and was held at 1000 psi for three minutes. The cavity was thenopened and the clad molded composite was blown out with a jet of air.The resultant copper-clad composite was etched approximately to yield atest pattern upon the copper surface. The copper-clad composite was thentested for various electrical, mechanical and physical properties, whichproperties are summarized in Table IV.

                  TABLE IV                                                        ______________________________________                                        Flexural Strength   23 × 10.sup.3 psi                                   (ASTM D790)         (average of 5 values)                                     Flexural Modulus    1.7 × 10.sup.6 psi                                  (ASTM D790)                                                                   Solder Float        no blistering                                             (floated on 500 degrees F.                                                    molten solder for 20 seconds)                                                 Peel Strength       4 lbs/inch                                                                    conductor width                                           Insulation Resistance                                                         After 11 days       2.1 × 10.sup.5 megohms                              Water Absorption    0.3 percent                                               (after 24-hour immersion in                                                   tap water at room temperature)                                                Profilimeter        15 microinches rms                                        Glass Transition Temperature                                                                      151 degrees C.                                            ______________________________________                                    

I claim:
 1. A process for producing conductor-clad printed wiring boardcomposites comprising,placing a quantity of substantially uncuredpolymeric molding compound in a heated compression molding apparatus;compression molding and curing the compound; the invention characterizedin that (a) the linear dimensions of the molding compound charge placedwithin the molding apparatus are less than the interior lineardimensions of the mold face upon which it is placed, (b) a conductivefoil of thickness less than 7 mils is placed upon, and supported by, themolding charge, prior to molding, (c) the linear dimensions of theconductive foil are less than the interior linear dimensions of the moldface, but greater than the linear extent of the molding compound charge,(d) the compound is caused to flow during the molding process, (e) themolding compound is constrained by the mold walls when the mold isclosed, and (f) the foil is smooth to within 20 microinches rmssubsequent to molding, whereby a conductor-clad composite is formed. 2.The process of claim 1 wherein the conductive foil is a copper foil. 3.The process of claim 1 wherein the molding compound comprises(1) between60 and 80 parts by weight thermosetting polymeric resin; (2) between 0.5and 60 parts by weight monomer; (3) between 0.5 and 3 parts by weightcatalyst; and (4) between 100 and 200 parts by weight filler.
 4. Theprocess of claim 3 wherein the molding compound further comprises atleast 20 parts by weight discrete reinforcement filaments less than 4inches in length.
 5. The process of claim 1 wherein the molding compoundis a sheet molding compound.
 6. The process of claim 1 wherein themolding compound is a bulk molding compound.
 7. The process of claims 2through 4 wherein the molding compound placed within the mold coversbetween 30 and 90 percent of the mold surface when inserted.
 8. Theprocess of claim 7 wherein the linear dimensions of the copper foil arebetween 99.9 and 95 percent of the internal linear dimensions of themold face upon which it is placed.
 9. The process of claim 8 wherein acopper foil is placed on an interior face of the mold prior to placementof the charge of molding compound and a second copper foil is placedupon the molding compound subsequent to the placement of the compoundwithin the mold and prior to compression of the molding compound. 10.The process of claims 2 through 4 wherein the compound comprises a resinwhich is selected from the group consisting of isophthalic anhydridebased unsaturated polyester resins, vinyl ester based resins,orthophthalic anhydride based unsaturated polyester resins and bisphenolbased polyester resins.
 11. The process of claim 4 wherein the compoundfurther comprises from 15 to 40 parts by weight low profile additive.12. The process of claim 11 wherein the low profile additive is apolyvinyl acetate low profile additive.
 13. The process of claim 11wherein the monomer is styrene.
 14. The process of claim 13 wherein thecompound further comprises 0-5 parts by weight viscosity control agent.15. The process of claim 14 wherein the viscosity control agent ismagnesium oxide.
 16. The process of claim 15 wherein the conductive foilis a copper foil.
 17. The process of claims 2 and 16 wherein a circuitpattern is defined on the conductive foil subsequent to the molding.